Gas discharge tubes, such as familiar "neon" tubes, are widely used in signs, works of art and for illumination. Such tubes are made by evacuating the air from a glass tube and then introducing a selected gas such as neon inside the tube. The tube is sealed at each end around an electrode with external connection wires. Conventional neon tubes use ordinary 115VAC, 60 Hz electrical power. In order to provide enough voltage to ionize the gas, a current limiting type of transformer is employed to step up the 115V to the level of usually 2000V to 12000V, depending on tube length. The pressure of a gas in the tube is usually from about 1 to about 15 mm Hg. Instead of neon, the tube may contain other noble gases, mercury vapor of other suitable known gases or gas mixtures, and the tube may be transparent, translucent or internally coated with a material that fluoresces when subjected to the radiation of the gas discharge. Different gases and coatings produce different colors and intensities of light. Most of the familiar gas discharge tubes produce a steady continuous line of illumination along their lengths.
It is known, as disclosed in Kayser U.S. Pat. No. 1,939,903, that a gas discharge tube can be operated to produce illumination that starts at one electrode and appears to grow from that electrode through the tube and toward the other electrode. This effect is obtained by adjusting the frequency or voltage, or both, of the exciting current. The Kayser patent also discloses that the tuning element, disclosed to be a variable condenser, can be adjusted so that the illumination in the tube has a "bead and nodule" effect and that fine tuning of the condenser can cause the beads to move in one direction or the other through the tube, or can cause the beads to appear to remain stationary within the tube.
U.S. Pat. No. 2,091,953 issued to Becquemont discloses that discontinuous illumination of a gas discharge tube can be caused to move within the tube by superimposing a direct current on the alternating current that ionizes the gas. Other prior art such as Seaman et al. U.S. Pat. No. 2,121,829 and Skirvin U.S. Pat. No. 3,440,488 also disclose either processes for making the illuminated portion of the gas in a tube grow along the length of the tube to give the appearance of writing or to have dark spots move through an illuminated tube progressively from one end to the other.
Although it is known how to produce discontinuous illumination in a gas discharge tube and how to cause discontinuities, which have the appearance of bubbles to move one way or the other by superimposing direct current through the tube, there is great difficulty in regulating the movement of the discontinuities o bubbles. The motion of the bubbles is extremely sensitive to small changes in the direct current so that the apparent velocity of bubble movement or even the direction of bubble movement will change drastically if a small change in applied direct current flow occurs.
The electrical energy driving a gas discharge tube is susceptible to many small modifications. The energy coming from a power line varies somewhat in voltage from time to time during an ordinary day, there are seasonal and daily temperature variations that influence the circuit in which a gas discharge tube is operated and there are imperfections in the tube electrodes and in other portions of the circuit which will cause changes in the amount or quality of direct current in the circuit as a function of time or temperature. In fact, because electrodes are not identical any gas discharge tube will act to some extent as a rectifier, thereby producing its own direct current which will vary with time and changes in ambient temperature conditions. As a result, existing devices are not capable of maintaining bubble motion at a desired velocity and direction unless all electric conditions within the circuit affecting bubble motion are closely controlled.